Question

I define a class with abstract type as follow:

abstract class AbsCell2{
    type T
    val init: T
    private var value: T = {
         println("Hello "+init);
         init
    }
    def get : T = value
    def set(x : T) = { value = x}
}

Now I instantiate an object with type Int

scala> val cell = new AbsCell2{type T = Int ; val init = 10}
Hello 0
cell: AbsCell2{type T = Int} = $anon$1@969ebb

Pay attention to the output from println. It seams the variable init hasn’t been initialized as 10. Note that the version of scala is 2.9.0-1

Answer 1

I think you’re looking for Scala’s early initializers,

scala> val cell = new {type T = Int ; val init = 10} with AbsCell2
Hello 10
cell: AbsCell2{val init: Int; type T = Int} = $anon$1@1efa9557

scala> cell.get
res0: cell.T = 10

Early initializers allow you to allocate a new object and set some specific fields before the class constructor runs. In this case, since value depends on init, we use the early initializer syntax (new { val init = ... } with AbsCell2) to first set init so that the class constructor can properly initialize value.

See also this question: In Scala, what is an "early initializer"?